The former couple were "hugging and kissing," an eyewitness told Celebuzz.
Earlier this year, multiple sources told Us that Brown, 23, and Rihanna, 24, have been secretly hooking up. The on-and-off twosome split in February 2009 following Brown's brutal assault of the "We Found Love" singer.
At the 2012 MTV Video Music Awards on Sept. 6, Brown publicly congratulated Rihanna on her Video of the Year win by kissing her on the lips.
Brown's girlfriend of nearly a year, aspiring model Karrueche Tran, 23, was "totally pissed off," a source told Us. "She felt the kiss was a public slap in the face."
On Monday, Tran tweeted, "There's a difference between a man and a boy." She added, "I prefer men."
This article originally appeared on Usmagazine.com: Chris Brown, Rihanna Make Out at NYC Club